Small ability proposal for the Witch-King. Literally just two abilities.
The two abilities I see the Witch-king getting are Ice Manipulation and Weather Manipulation. Let me explain.
According to the men of Lossoth, the Witch-king is capable of making frost and thaw at will:
“For a while Arvedui hid in the tunnels of the old dwarf-mines near the far end of the Mountains, but he was driven at last by hunger to seek the help of the Lossoth, the Snowmen of For-ochel. Some of these he found in camp by the seashore; but they did not help the king willingly, for he had nothing to offer them, save a few jewels which they did not value; and they were afraid of the Witch-king, who (they said) could make frost or thaw at his will.” - The Return of the King, Appendix A, I The Númenorean Kings, (iii) Eriador, Arnor, and the Heirs of Isildur
This is confirmed yet again by the men of Lossoth, where the Witch-king sent a storm that apparently sunk:
“But the Snowmen were uneasy: for they said that they smelled danger in the wind. And the chief of the Lossoth said to Arvedui: ‘‘Do not mount on this sea-monster! If they have them, let the seamen bring us food and other things that we need, and you may stay here till the Witch-king goes home. For in summer his power wanes; but now his breath is deadly, and his cold arm is long. But Arvedui did not take his council [...] Yet the counsel of the Lossoth was good, by chance or by foresight; for the ship had not reached the open sea when a great storm of wind arose, and came with blinding snow out of the North; and it drove the ship back upon the ice and piled ice up against it. Even the mariners of Círdan were helpless, and in the night the ice crushed the hull, and ship foundered. So perished Arvedui Last-king, and with him the palantíri were buried in the sea.’’ - The Return of the King: Appendix A